Good human vision depends upon properly focusing light incident on the front of the eye onto the retina. To this end the human eye includes an internal lens, and muscles that exert forces on that lens. In youth these muscles can change substantively the focal length of the internal lens, thereby enabling focusing on objects at various distances. With increasing age the ability of these muscles to affect focus decreases until, after about forty-five, most people can no longer focus at both far and near . . . even after any innate hyperopia, or myopia, and/or astigmatism is corrected. Optometrically speaking, people evidencing this condition are called ‘presbyopes’.
Variable focus eyeglasses have been devised that enable a presbyopic wearer manually to adjust focal length, and thereby to focus—on demand—at any distance. See Kurtin, U.S. Pat. Nos. 5,138,494 and 5,668,620 and US Patent Application Publication No. 2008/0084532. The eyeglasses disclosed in the aforementioned patents typically include a) for each eye a fixed optical power equal to the intended wearer's required refractive correction for far vision, together with b) a manually adjustable, bilaterally tracking, spherical ADD power. The maximum ADD can, in some instances, be as large as about 2.75 diopters, which amount is sufficient to permit a change in focus from far viewing to somewhat closer than ‘reading distance’ (usually assumed to be about 16 inches). Emerging presbyopes {those who retain some natural focusing ability} usually require a lesser amount of maximum ADD.
Adjustable focus eyeglasses as just described will suit most presbyopes. Some, however, suffer also from fluctuating vision. This means that the spherical optical power required to correct their far vision varies from time to time (i.e., from hour to hour, or from day to day). Fluctuating vision can arise as a consequence, for example, of diabetes, dry eye, hyperthyroidism, menopause, or possibly other conditions.
While statistically comprehensive information is seemingly not available, published clinical data indicates that in many instances the fluctuation is primarily as to sphere, and can exceed two diopters. More to the point, when an individual with fluctuating vision presents him- or herself for a refraction there is often no conclusive way in which the optometric practitioner can be confident as to i) the relevant range of fluctuation, ii) whether or not that range is static, and iii) where within that range the patient's vision is just then situated. Hence what is needed is eyeglasses which will allow the user, from time to time, to modify his/her spherical refractive correction to offset corresponding biological fluctuations.